1. Field of the Invention
The present invention relates to a semiconductor apparatus. More specifically, it relates to a semiconductor apparatus equipped with an internal circuit including a semiconductor element and a protection circuit including a semiconductor element for protecting the internal circuit against damage from electrostatic discharge (ESD).
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
A semiconductor integrated circuit includes semiconductor elements, such as, e.g., MOS transistors. Prevention of electrostatic discharge (ESD) damage due to a pulse high voltage generated by electrostatic discharge is a serious concern in such a semiconductor integrated circuit. To prevent such damage, various protection methods have been proposed.
FIG. 1 shows a typical semiconductor integrated circuit equipped with an internal circuit 201 and a protection circuit 101 for protecting the internal circuit against damage from electrostatic discharge (ESD). The protection circuit 101 includes a p-type protection element 102 and an n-type protection element 103 connected between a power source terminal Vdd and a ground terminal GND. An input terminal Vin is connected to a drain connection of the p-type protection element 102 and the n-type protection element 103 constituting the protection circuit 101. Furthermore, the input terminal Vin is designed such that it can be connected to the drain connection of (or source connection) of a p-type internal circuit element 202 and an n-type internal circuit element 203 constituting the internal circuit 201. Through the drain connection (or source connection), an input signal is transmitted into the internal circuit 201.
Conventionally, even in cases where design conditions of the p-type protection element 102 and the n-type protection element 103 are identical to those of the p-type internal circuit element 202 and the n-type internal circuit element 203, the protection circuit 101 could have protected the internal circuit 201 against damage from electrostatic discharge (ESD).
In accordance with the recent advancement of high integration of semiconductor apparatuses, the operation voltage has been lowered and the power consumption has been lowered, and the semiconductor element constituting the semiconductor apparatus has been miniaturized in structure and increased in density. As a result a semiconductor apparatus, especially MOS transistors, have become easily damaged by ESD. Accordingly, the aforementioned protection method has become insufficient against damage from electrostatic discharge (ESD).
In view of the aforementioned problems, it has been proposed that the protection circuit 101 is designed so as to easily release electrostatic stress current by differentiating the design conditions of the semiconductor elements constituting the protection circuit 101 from those of the semiconductor elements constituting the internal circuit 201. For example, Japanese Unexamined Laid-open Patent Publication No. H05-75118 proposes that the p-type protection element 102 and the n-type protection element 103 constituting the protection circuit 101 are set to be shorter in channel length than the p-type internal circuit element 202 and the e-type internal circuit element 203 constituting the internal circuit 201 to enhance the electrostatic stress release function of the protection circuit 101.
The technical means of this proposal, however, has the following technical drawbacks. That is, the limit of shortening the channel length of each of the p-type protection element 102 and the n-type protection element 103 constituting the protection circuit 101 is decided by the durability of hot electron. In other words, the channel length should be set to be longer than the lower limit channel length below the rated voltage due to the lowered voltage endurance by the sudden punchthrough. It is not practical to unlimitedly shorten the channel length. In accordance with the recent high integration of a semiconductor integrated circuit, however, the p-type internal circuit element 202 and the n-type internal circuit element 203 constituting the internal circuit 201 have been shortened in channel length to the aforementioned lower limit. Under the circumstances, there are very limited cases in which the p-type protection element 102 and the n-type protection element 103 constituting the protection circuit 101 can be shorten in channel length than the p-type internal circuit element 202 and the n-type internal circuit element 203 constituting the internal circuit 201.
The same protection effects can be obtained by setting the p-type internal circuit element 202 and the n-type internal circuit element 203 constituting the internal circuit 201 to be longer in channel length than a conventional element. This is, however, impractical since the channel length of the p-type internal circuit element 202 and that of the n-type internal circuit element 203 are decided by the circuit use conditions.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.